The present invention relates to an assembly for connecting the wooden members of a truss using nail plates and adhesive to obtain greater strength and stability.
Conventional nail plate trusses are made from selected grades of softwood lumber assembled with toothed metal plates which are driven into the lumber at all junction points with hydraulic or roll pressure. The lumber is chosen by grade to have appropriate strength for the function it performs in the truss, such as top chord, bottom chord and web members. The steel in the galvanized metal toothed plates is likewise selected and controlled for the highest strength level which still permits die-punching and bending to form the individual teeth. By these material selection processes, the building loads which the trusses will carry are maximized for a given size of lumber and truss span.
Trusses designed and assembled in this manner are vulnerable to loss of load-bearing capability by any movement of the wood members which tends to loosen the embedded metal teeth of the nail plates. This kind of movement includes lateral flexing of the trusses during transport, unloading, job-site handling and erection. It also includes loosening due to changes in moisture content of the lumber by repeated cycling through a wide range of seasonal humidity changes or simply drying of the lumber from an original green (or wetted) condition to heated inside dry conditions.
The strength of nail plate trusses is also limited by the extreme pressure placed on the metal teeth of the nail plates at certain points of the truss assembly, particularly at the outboard corners or heels where the plates are subject to great shear and rotational forces as the truss is loaded. Thus, the force on these heel plates which tends to rotate the metal teeth out of the wood actually limits the allowable load which the truss can carry. Similarly, limiting forces also occur in tensile joints in the bottom truss chord and in certain other multiple point truss member connections.
Before the metal nail plate concept and assembly came into general use, trusses were most often assembled with nailed and glued plywood gusset plates at all points of juncture. The strength of the glued wood-to-wood bond provided a very substantial contribution to the load bearing capability of the nails in the wooden gusset plate. The glue bond contributed not only resistance to joint separation due to lateral flexure during the handling and erection but also substantial load-bearing strength to the joint itself, especially in resisting shear and tensile forces among the elements of the truss. Moreover, the nails in the gusset plates could be omitted entirely if the joints were left under pressure long enough to completely cure the glue while maintaining continuous surface contact. As well as these gusset-plated trusses performed, they have been largely replaced by toothed metal plate trusses on the basis of speedier and easier assembly resulting in lower costs.